Conventionally, in a control device for a combustion furnace, or the like, a combustion sequence is controlled based on a clock signal of a CPU (See, for example, Japanese Unexamined Patent Application Publication H08-247455). In this combustion sequence there are timings that are important to safety, such as the pre-purge timing, the ignition timing, the flame response, and the like. If the clock is changed through, for example, a failure in the clock source for the CPU, the timing of the combustion sequence can be outside of specifications, and non-combusted gases may accumulate or there may be excessive gas flows, beyond the explosive threshold, leading to the risk of explosion. Given this, in recent years there has been a proposal for detecting failures in the clock source by providing redundant clock sources through the provision of two CPUs, to perform mutual monitoring of time data.
However, the crystal oscillators that are typically used as clock sources, if manufactured using identical processes, have extremely similar temperature characteristics and useful life characteristics. Consequently, because the frequency would fall if, for example, placed in a high temperature environment, the frequency would fall by the same amount if placed in the same environment, and so if identical products are used for the redundant clock sources, then identical failures may be produced by identical factors, making it difficult to detect faults.
Given this, the object of the present invention is to provide a control device capable of detecting faults in a clock source more accurately.